Grain-oriented electrical steel sheets are mainly utilized as iron cores for transformers and are required to have excellent magnetic properties, in particular low iron loss.
In this regard, it is important to highly accord secondary recrystallized grains of steel sheets with the (110)[001] orientation (or so-called Goss orientation) and reduce impurities in product steel sheets.
However, there are limitations in controlling crystal orientation and reducing impurities in terms of balancing with manufacturing cost, and so on. Thus, a method of applying linear strain to grain-oriented electrical steel sheets to narrow magnetic domain widths and reduce iron loss, is well known.
Techniques to narrow magnetic domain widths and improve iron loss properties as described above include a non-heat resistant magnetic domain refining method where a thermal strain region is linearly disposed (e.g., refer to JP 557-2252B or JP H06-72266B) and a heat resistant magnetic domain refining method where a linear groove with a predetermined depth is disposed on the steel sheet surface (e.g., refer to JP 562-53579B or JP H03-69968B).
JP S62-53579B discloses a means of forming a groove by using a gear type roller, and JP H03-69968B discloses a means of forming a groove by pressing an edge of a blade against a steel sheet after final annealing. These means are advantageous in that the magnetic domain refining effect on the steel sheet does not dissipate through heat treatment and that they are also applicable to wound iron cores and the like.
We found the following problems.
First, in conventional non-heat resistant magnetic domain refining methods such as disclosed in the aforementioned JP S57-2252B and JP H06-72266B, formation of a base film on the floor of a groove is insufficient and, therefore, tension received from the base film or the insulating tension coating is made insufficient in the groove part and steel substrate in the vicinity thereof. Because of this, sufficient iron loss reduction effect could not be obtained in many cases.
On the other hand, in heat resistant magnetic domain refining methods such as disclosed in the aforementioned JP S62-53579B or JP H03-69968B, fine grains are generated under the groove through flattening annealing due to strains formed in mechanical working. If the fine grains exist in an appropriate amount, they would contribute to magnetic domain refining and exhibit an effect of reducing iron loss. However, it is difficult to appropriately control the generation amount of fine grains. Further, if there is a large generation amount, magnetic permeability deteriorates and a desirable iron loss reducing effect cannot be obtained.
Another method of forming a groove is a method such as the so-called etching where insulating coating is removed linearly during or after final annealing (e.g., refer to JP S62-54873B). However, with that method, there was a problem in that because of the absence of a base film in the groove part, disturbances in the magnetic domain tend to occur in the vicinity of the groove part and, therefore, iron loss is not sufficiently improved.
It could therefore be helpful to provide a grain-oriented electrical steel sheet having low iron loss properties by applying magnetic domain refining treatment to a grain-oriented electrical steel sheet by forming a groove by a chemical means, and an advantageous manufacturing method of obtaining such steel sheet.